Inclusion Criteria:

• Patients must meet DSM-IV diagnostic criteria for ADHD based on K-SADS-PL prior to randomization.
• Patients must have an ADHDRS-IV-Parent: Inv total score at least 1.5 standard deviations above the age/gender norm prior to randomization. They must have a CGI-ADHD-S score greater than or equal to 4 at both the patients screening visit, prior to randomization.
• Patients must have laboratory results; showing no clinically significant abnormalities.
• Patients must be of normal intelligence, as assessed by the investigator.
• Patients/parents must have been judged by the investigator to be reliable to keep appointments for clinic visits and all tests, including venous punctures and examinations required by the protocol.
• Patients of child-bearing potential agree to use a reliable method of birth control during the study and for 1 month following the last dose of study drug. Female patients of child-bearing potential must test negative for pregnancy at the time of enrollment based on a urine pregnancy test.

Exclusion Criteria:

• Patients who weigh less than 18 kg or greater than 75 kg at screening and at randomization.
• Female patients who are pregnant or who are breast-feeding. Patients who have a history of Bipolar I/ II, psychosis, or pervasive developmental disorder.
• Patients who have current motor tics or a diagnosis of Tourette's Syndrome.
• Patients with marked anxiety, tension, and agitation sufficient, to contraindicate treatment with extended-release methylphenidate.
• Patients with a history of any seizure disorder, known electroencephalographic (EEG) abnormalities in the absence of seizures.
• If the ECG assessed at screening/prior to randomization shows an abnormality meeting one or more of the absolute exclusion criteria listed in the Pediatric ECG Alert Criteria must be excluded from the study.
• Patients who, in the opinion of the investigator, are at serious suicidal risk.
• Patients with a history of severe allergies to more than one class of medications, multiple adverse drug reactions, or known hypersensitivity to extended-release methylphenidate.
• Patients with a history of alcohol or drug abuse within the past 3 months prior to, or who are currently using alcohol, drugs of abuse, or any prescribed or over-the-counter medication in a manner that the investigator considers indicative of abuse.
• Patients who screen positive for drugs of abuse not prescribed by a physician cannot participate. Drug screen may be repeated at the discretion of the investigator, and the patient may be allowed to enter the study if the repeat screen is negative. All patients must have a negative drug screen before enrollment in the study.
• Patients who have a medical condition that would increase sympathetic nervous system activity markedly, or who are taking a medication on a daily basis that has sympathomimetic activity are excluded. Such medications can be taken on an as-needed basis.
• Patients with problems that would be exacerbated by increased norepinephrine tone, including a history of cardiovascular disease, thyroid dysfunction, glaucoma, urinary retention, or severe gastrointestinal narrowing.
• Patients who, at any time during the study, are likely to need psychotropic medications apart from the drugs under study.
• Patients who, at any time during the study, are likely to begin structured psychotherapy aimed at ADHD symptoms are excluded. Psychotherapy initiated at least 1 month prior to screening is acceptable.
• Patients who have used a monoamine oxidase inhibitor (MAOI) during the 2 weeks prior to randomization.
• Patients with current or past history of clinically significant hypertension.
• Patients who are currently enrolled in, or discontinued within the last 30 days from a clinical trial involving an off-label use of an investigational drug, or concurrently enrolled in any other type of medical research.
• Patients who have participated in a prior study of LY2216684.